1. Field of the Invention
The present invention refers to an endless belt conveyor for vegetable or mineral bulk products, and more particularly to an endless belt conveyor for the formation of a closed loop in a fixed or movable crushing unit. The invention is further directed to a crushing unit comprising the belt conveyor.
2. Description of the Related Art
Crushing units are well known, particularly those mounted on a vehicle chassis, comprising belt conveyors which are respectively and operatively associated with a crusher and with a classifying screen, for allowing the material, which is rejected by the classifying screen and passed through the crusher, to be reconducted to the screen, in order to be submitted to a new classification. This system of returning the crushed material to the classifying screen is generally denominated closed crushing loop and is schematically illustrated in FIGS. 1 and 2 of the enclosed drawings.
In the movable crushing units mounted on a vehicle chassis V, the bulk material to be crushed is directly or indirectly fed to a first belt conveyor 10 which conducts the material to a classifying screen 20, for example a vibrating screen, in which it is separated. The material passing through the classifying screen 20 is collected, for example in a hopper 21 and conducted to a discharge conveyor 22, which conducts the material passing through the classifying screen 20 to a storage heap S or any other adequate collecting means.
The large material rejected by the classifying screen 20 is conducted to a crusher 30, so as to be reduced to the desired size of the crushed product. The material released from the crusher 30 is fed to a second return belt conveyor 40 arranged beside the first belt conveyor 10, but which conducts the crushed material upwardly and in an opposite direction to the conduction direction of the first belt conveyor 10, so that it may release the crushed material at a height sufficient to allow the transfer of the material, by means of an inclined chute 50, to the first belt conveyor 10, which conducts the material back to the classifying screen 20, completing the closed crushing loop. It should be noted that the feeding of bulk material to the crushing unit can be made in the first belt conveyor 10, for example in the region where the crushed material is received, or in the second belt conveyor 40.
In these crushing units, the length of the second belt conveyor should be dimensioned to position its discharge end at a height much higher than that of the feed end of the first belt conveyor 10, in order to allow the transfer of the crushed material coming from the crusher 30 and to be returned to the classifying screen 20. In the illustrated construction, in which the first and the second belt conveyors 10 and 40 are arranged side-by-side, the transfer is carried out through the inclined chute 50.
The above-described closed loop has the function to reduce all the material sent to the crushing unit to a size that is smaller than the mesh of the classifying screen 20. This arrangement is applied to both the movable units and to the fixed units and it is particularized in that the return of the material to the classifying screen 20 is accomplished by means of two belt conveyors disposed side by side and moved in opposite directions.
One of the disadvantages of this solution refers to the height that the discharge end of the second belt conveyor 40 should present and which should be much higher than the height of the feed end of the first belt conveyor 10.
In order to avoid the material being carried by the second belt conveyor 40 from sliding in a direction opposite to that of the conveying run, the slope angle of the second belt conveyor 40 must be limited, usually not higher than 20 degrees.
The closed loop, as described above, requires a very long second belt conveyor 40, which besides increasing the cost in the case of movable units on wheels, impairs the displacement of these crushing units on highways.
The first and the second belt conveyors 10 and 40 on being arranged side by side increase the width of the crushing unit, exceeding the limit dimensions for highway transportation, requiring either the partial disassembly of the unit, or special authorizations to travel on highways.